The numerous options that exist today for processing of composite materials has led to diversified technologies. Autoclave processing is most widely used in the aerospace industry. Though this process is generally considered as slow and expensive, it allows the production of parts of various dimensions and shapes for airframe components under closely controlled temperature and pressure conditions.
Autoclave processing conventionally requires use of a tool or mold which supports and/or shapes the composite part in its formation. Thus, there is an initial lay-up of prepreg on the tool, a breather layer is placed over the prepreg and the sandwich inserted in a bag in the autoclave. The bag is evacuated and heat and pressure applied until the part is cured after which it is allowed to cool before removal.
The tool plays an important role in the autoclave process because it has a direct bearing on the quality of the part to be produced. The new high temperature matrix resins used in composites, such as bismaleimides, polyimides and thermoplastics (PEEK) are gaining increased importance in applications requiring high service temperatures. Tools for manufacture of parts from these composites should (1) be capable of repeated cycling above 350.degree. F.; (2) be dimensionally stable at the elevated temperatures; (3) be economical to make and to use, i.e., fast heat-up, and (4) have a low coefficient of thermal expansion that substantially matches the parts to be fabricated.
Current tool materials, such as epoxy/graphite, machined bulk graphite, ceramics and metals are deficient in one or more important respects. The epoxy/graphite tools do not meet the high temperature requirements for processing composites wherein the matrix resin is bismaleimide, polyimide or PEEK. Monolithic graphite is deficient as a tooling material primarily because of its limited durability. It may be used as a master tool but for repeated or multi-use it exhibits many deficiencies. A major concern with ceramic molds is their large thermal mass requiring high heat input to achieve the desired temperature. Among other things, this tends to cause degradation of the vacuum bag and sealant materials. Finally, the most extensively employed tooling materials are metals such as steel, aluminum and nickel. The biggest negative for the metals is their high coefficient of thermal expansion. The mismatch in thermal expansion between metal tools and composite parts promotes distortion of parts. As a result, steel tools are considered adequate only for relatively flat or slightly curved parts which are trimmed to final dimension after cure.
The present invention provides a tool which overcomes most if not all of the aforementioned deficiencies of the prior art tools and possesses still other advantages.